Strip mining is a method of removing the waste rock from above the ore body in strips to expose the ore to be mined and depositing the waste rock in the void of the previously mined strip. The strip mining method usually starts with a box cut, which removes the waste rock from above the ore body to another location. The ore is mined and the miners move to another strip. With the new strip the waste rock is deposited in the void created from the previously mined strip. The method is repeated as the mining operation continues to advance. For most strip mining, waste rock is blasted before removal but in some areas it is removed using earthmoving equipment large enough to penetrate the material without blasting.
The pit design for strip mining is usually a long advancing face uncovering the ore with a dump area following the advancing face. As the material is removed from the advancing face it is placed into a dump area void that was created from the previous advancing face.
The strip mining method often employs different equipment to remove rock waste. It is common in large operations that before or after blasting, a pre-strip cut is made by a truck and shovel fleet and or a dragline will take multiple passes to move the waste rock into the dump position. In some mines, dozers are used to assist the dragline in removing the waste rock. In some mining operations dozers are used as a primary digging tool and they have a dedicated fleet of dozers for the dozer push task.
Pit design takes into consideration what equipment is used for the mining method. The pit design for strip mining in many cases has the waste rock moved directly across the pit when draglines are used. When a truck and shovel operation is used the waste rock must be carried out of the pit and along roadways. The loaded trucks can only travel up roadways that are not too steep.
Strip mining has evolved and one of the most effective earth moving equipment in strip mining has become the bulldozer. The method of moving earth with a bulldozer is commonly referred to as dozer push. However dozers are limited to pushing short distances and are less effective when pushing uphill as opposed to pushing downhill. Dozing is generally limited to a 25% to 30% grade to push material uphill.
More commonly the truck and shovel are used to remove waste rock. The shovel may have an excavator configuration or a face shovel configuration. Its operation is restricted to a grade that the trucks can drive up when fully loaded. This usually means the trucks must drive from the pit where they are loaded, up a ramp and onto the waste dump. As a general rule, trucks are limited to traveling up a 10% uphill grade when fully loaded. The area in which they are loaded and the area in which they are unloaded generally needs to be flat. This increases the length of travel needed to lift any load upwards at a 10% grade.
As a comparison, dozers need less distance to lift material to a given height than a truck. However if the lift height becomes too great, the dozer loses its advantage because it becomes less effective with increasing push lengths. Dozers do have another advantage in that they do not need a flat area in which to load or unload. This reduces the total length needed to lift material when compared to trucks.
Both dozer and truck/shovel mining methods have increased costs when the material has to be lifted higher to the waste dump area. It is therefore an advantage to reduce the lift height of the waste material. The decision to use dozers is one of commercial feasibility and depends on a number of factors including the ore body position (including length and depth) and steepness of gradients.